In recent years, there have been developed vehicles such as hybrid vehicles that drive using, in whole or in part, the electric power supply from the battery as a power source. Most of these vehicles have a power supply system that converts a direct current power supplied from a battery to an alternating current power using an inverter or other electric power conversion circuit, and supplies the converted alternating current power to an AC motor or other loads.
The battery used in the power supply system provides a high voltage and has a large capacity. Thus, if an electrical leakage arises in any part of the electric circuit, there is a possibility of such trouble as an electric shock to an engineer who performs maintenance of vehicle. For this reason, it is required for the vehicle-mounted power supply system to detect the electric leakage as soon as possible, and in case of detecting the leakage, a necessary countermeasure should be immediately taken.
FIG. 6 is a diagram showing a conventional leakage detection device of the vehicle-mounted power supply system. Such leakage detection device is disclosed in the Patent Literature 1 below.
In FIG. 6, the leakage detection device comprises a power supply system 10 and a leakage detection section 20.
The power supply system 10 comprises a DC high voltage circuit A and an AC high voltage circuit B. The DC high voltage circuit A comprises a battery 11 for the direct current, a positive line 13 and a negative line 14 that are connected respectively to positive and negative sides of the battery 11, and contactors 17a, 17b that are provided on the positive line 13 and the negative line 14, the contactors 17a, 17b being connected respectively to the positive line 13 and the negative line 14. The AC high voltage circuit B comprises an inverter circuit 12 that is connected to the positive line 13 and the negative line 14 and converts the direct current power to the alternating current power by switching on/off plural switching elements, an AC motor 15, and plural AC lines 16 that connect the inverter 12 with the AC motor 15.
To activate the AC motor 15, the contactors 17a, 17b are turned on.
The inverter 12 is, for example, an IGBT inverter circuit 12 as shown in FIG. 7. The IGBT inverter circuit is provided with six IGBT elements (switching elements) 76 and six IGBT circuits 70-75 having corresponding six diodes 77.
When the AC motor 15 is a three-phase motor, three sets of circuits, the IGBT circuits 70, 73, the IGBT circuits 71, 74 and the IGBT circuits 72, 75, are connected in parallel. Additionally, an intermediate point M1 between the IGBT circuits 70, 73, an intermediate point M2 between the IGBT circuits 71, 74 and an intermediate point M3 between the IGBT circuits 72, 75 are respectively connected to three coils in the AC motor 15.
The leakage detection section 20 comprises a capacitor C that is connected to a voltage applying point P on the positive line 13 connected to the positive side of the battery, a resistance R that is connected to the capacitor C, an oscillator 21 that generates an AC signal Vs with a prescribed frequency such as a sine wave or a square wave and provides the AC signal Vs to the resistance R, and a voltage measurement section 40 that measures a voltage level (effective value of the AC voltage) at a voltage measurement point Q located between the resistance R and the capacitor C. While the voltage measurement section 40 measures the voltage level, a threshold value is set to determine whether or not the leakage exists.
A process of detecting the leakage in the leakage detection section 20 shown in FIG. 6 is performed as follows.
It is assumed that the insulation of the negative line 14 becomes deteriorated and leakage occurs therein.
The AC signal Vs outputted from the oscillator 21 passes through the resistance R and the capacitor C, and is applied to the applying point P on the positive line 13.
If no leakage exists in the power supply system 10, the effective value of voltage measured by the voltage measuring section 40 is substantially the same as the effective value of voltage of the AC signal Vs outputted from the oscillator 21, and equals or exceeds a set threshold value. Accordingly, it is determined that the leakage does not exist.
On the other hand, if the leakage exists in the power supply system 10, in other words, if the leakage exists in the negative line 14, there occurs a leakage resistance r between the negative line 14 and the body (earth) of the vehicle. Thus, the effective value of voltage of the AC signal Vs is divided to the resistance R and the leakage resistance r. Therefore, the effective value of voltage measured with the voltage measuring section 40 is smaller than the effective value of voltage of the AC signal Vs outputted from the oscillator 21, and is lower than the set threshold value. Accordingly, it is determined that the leakage exists. As described above, by measuring the voltage at the measurement point Q and comparing the measured voltage with the threshold value, it can be detected whether or not the leakage exists.
Patent Literature 1: Japanese Patent Application Laid-open No. 2003-219551